ABV from gravity calculator
Alcohol strength from original and final gravity.
Fill in the fields and the result will appear here automatically.
Compares specific gravity before and after fermentation and estimates ABV using a selected linear factor. SG is density relative to water: 1.050 and 1.010 are neither sugar percentages nor degrees Plato. The neighbouring attenuation result is apparent attenuation, the relative fall in SG above water’s reference value. Alcohol also changes density, so this result does not measure the fraction of sugar actually consumed, residual sweetness or whether fermentation has finished.
How it works
Formula and logic
ABV = (OG − FG) × factor; apparent attenuation = (OG − FG) ÷ (OG − 1) × 100%. The model requires OG > 1, 0 < FG ≤ OG and a positive factor. FG below 1 can give apparent attenuation above 100% without implying that more than all the sugar was consumed. An ABV estimate above 100% is rejected.
Example
OG 1.050, FG 1.010 and factor 131.25 give 5.25% ABV and 80% apparent attenuation. Unchanged FG 1.050 gives zero for both. FG 0.990 gives 7.875% ABV and 120% apparent attenuation.
Fields and units
- Original gravity — unitless
- Final gravity — unitless
- Conversion factor — unitless
How to use
- — Enter both gravity readings on the same SG scale.
- — Match the hydrometer’s calibration conditions; this tool does not calculate a temperature correction.
- — 131.25 is the retained linear factor; select another only as part of a stated method.
- — A single final reading does not establish completed fermentation. Distillation and laboratory analysis are outside this model.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- Brewfather: specific gravity, linear ABV estimation and apparent versus real attenuation; no accuracy guarantee
- Limitation
- Two SG readings provide an estimate, not laboratory-certified strength, a drink-safety assessment or proof of completed fermentation.
FAQ
Why does gravity fall during fermentation?
Fermentation changes the mixture: some extract is consumed and alcohol with a different density appears. A fall in SG helps estimate ABV but is not a direct mass measurement of consumed sugar.
What does attenuation tell me?
It is apparent attenuation, calculated as (OG − FG)/(OG − 1). Alcohol’s effect on density separates it from real attenuation; 80% is not a measurement of residual sugar.
Why do the factors differ?
A linear factor approximates a more complex relationship. Changing it changes estimated ABV, without making the result an exact measurement. This page has no established numerical accuracy guarantee.
Is a temperature correction needed?
Use the instrument’s calibration instructions to make both readings comparable. The calculator applies no automatic correction for measurement temperature.